This invention provides for improved production of high octane fuel from naphtha by hydrocarbon cracking to obtain C5-olefins, particularly isobutene and isoamylenes, for upgrading by etherification, alkylation or the like. In particular, it provides methods and catalysts for cracking C.sub.7 + paraffinic and naphthenic feedstocks, such as naphthenic petroleum fractions, under selective reaction conditions to produce etherifiable isoalkenes.
There has been considerable development of processes for synthesizing alkyl tertiary-alkyl ethers as octane boosters in place of conventional lead additives in gasoline. The etherification processes for the production of methyl tertiary alkyl ethers, in particular methyl t-butyl ether (MTBE) and t-amyl methyl ether (TAME) have been the focus of considerable research. It is known that isobutylene (i-butene) and other isoalkenes (branched olefins) produced by hydrocarbon cracking may be reacted with methanol, ethanol, isopropanol and other lower aliphatic primary and secondary alcohols over an acidic catalyst to provide tertiary ethers. Methanol is considered the most important C.sub.1 -C.sub.4 oxygenate feedstock because of its widespread availability and low cost. Therefore, primary emphasis herein is placed on MTBE and TAME and cracking processes for making isobutylene and isoamylene reactants for etherification.
Naphtha reforming, in which C.sub.6.sup.+ paraffins and naphthenes are converted to aromatics, provides a major source of high octane gasoline in current refining strategies. However, due to environmental constraints, replacement of aromatic-rich reformate with high octane ethers and alkylate would be highly desirable. Since these latter processes require feedstocks such as C.sub.3 -C.sub.5 olefins and isoparaffins, a means of selectively converting naphtha to lighter hydrocarbons would be very beneficial. A major drawback to the cracking naphtha to obtain these lighter feedstocks is the concurrent formation of light gas (C.sub.1 -C.sub.2) and linear C.sub.3 -C.sub.5 paraffins, which are low value by-products. Therefore, development of a catalyst which maximizes the yield of C.sub.3 -C.sub.5 olefins and isoparaffins during naphtha cracking would be highly desirable.